I have taken a year and a half break from riding. What can I do to prepare myself?

To be honest your body will react from that differenty, but im positive that once you get on that horse it may take a few days but ull get back to were you were very fast i was off a horse for a few years jumped back on and in a few days i was like a pro again. Best thing you can do is jump back on the saddle and go as they say. Good luck with that and hope to hear good news from it.